1. Field of the Invention
The invention concerns a method to compensate measuring errors due to temperature of an optical arrangement having at least one lens and a corresponding optical arrangement.
2. Description of Related Art
In numerous areas of industry such as the semiconductor and automobile industry, optical arrangements are widespread. They are, for example, used for contactless determination of positions or measurements and must work in temperature ranges from −30° C. to +70° C. while doing so in a reliable and precise manner. Such temperature changes, however, result in dimensional changes of the optical and mechanical components due to the temperature coefficient of expansion specific to a material. This considerably reduces the measuring accuracy. An aluminum bracket with a length of 10 mm, for example, expands with a temperature change from −20 to +40° C. by 14 μm. In order to compensate this temperature effect, various fixtures have already been proposed.
U.S. Pat. No. 4,919,519 discloses a system having a lens arrangement that is shifted in its position depending on the temperature. For this, a liquid is contained in a chamber, which moves the lens arrangement along the longitudinal optical axis in the event of expansion due to temperature. As a result, the optical arrangement is influenced in a focused manner depending on the temperature.
Another arrangement also drawing on the expansion of a liquid is shown in DE 10 2006 046 416 A1. There, an optical element is driven via a piston-cylinder unit relative to a casing depending on the temperature. In the process, a liquid with defined temperature coefficients is contained in the piston-cylinder unit. By means of suitable adjustment of the individual elements in the arrangement, thermal influences can be compensated to a considerable extent.
Another approach is demonstrated in DE 102 61 973 A1. There, a tube-shaped bracket for a lens is inserted in an intermediate bracket. The bracket and the intermediate bracket are selected in terms of their dimensioning and in the temperature coefficients of the materials such that the expansions of the bracket and intermediate bracket compensate one another in the event of a temperature change and the lens is kept in a constant position.
With the fixtures disclosed in the publications JP 58 00 7109 A and WO 2007/023052 A1, corrective materials with a defined temperature expansion coefficient are arranged between a detector element and the casing surrounding the arrangement, which subsequently adjust the position of the detector in the event of changes in temperature. As a result, the distance between the optics and detector can be kept constant with suitable dimensioning.
With the fixtures known on the basis of the state-of-the-art, the fact that the optical arrangements and their components must be adjusted with respect to one another to a considerable extent is problematic. Even if only individual components are to be replaced or if, as a result of production tolerances, fluctuations in the temperature coefficients occur, solely an insufficient compensation of the errors resulting from a change in temperature occurs. Such fixtures are not lastly associated with considerable costs as a result and exhibit a corresponding susceptibility to errors.
Furthermore, with the construction of the fixtures, one must often resort to special and therefore expensive materials in order to adjust the individual components with one another in a suitable manner. Furthermore, the additional components for the compensation of a change in temperature result in the enlargement of the fixture designs. In addition, such fixtures cannot be implemented or only to a very limited extent with optical arrangement, which exhibit gradient-index lenses. Gradient-index lenses exhibit a non-linear change in the refractive index of the lens in addition to the geometrical change in dimensions.
Therefore, it is the task of this invention to achieve and further embody a method and an optical arrangement of the type initially stated and to configure it further such that affordable and reliable compensation of measuring errors resulting from a change in temperature without any considerable increase in resources required for the construction can be achieved.